Trainer Matthew Smith is at the centre of an inquiry after syringes and unidentified substances were found in a horse float at Randwick.

Racing NSW's chief investigator Nathan Hayward found four syringes during a routine inspection of horse floats at Randwick for Saturday's race meeting.

Smith told stewards the substance was acepromazine, a tranquiliser, which he said was always in his truck.

The inquiry was adjourned for the substance to be tested.

Smith had one runner at the meeting, La Courbette, who finished sixth in the Group Three Aspiration Quality.